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Amplitude correction
Important methods:
• Correction for spherical Divergence
• AGC - Automatic gain control
• Programmable Gain functions
• Trace balancing
• Reflection and transmission at an interface
• Geometrical spreading
• Absorption
• Receiver response
• Measurement system
Loss of amplitude due to
Problem for Data processing
Individual large Amplitudes dominate the processing
Reflections are difficult to recognize
Strong amplitude contrasts influence the digital filtering (especially for large travel-times)
Common shot-gathers just after demultiplexing
Yilmaz, 1987
Advantage:Physical base for amplitude correctionRelative Amplitude difference remains equal
Disadvantage:Velocity function not known beforehandNoise sources can still remain dominant
Correction for spherical divergence
0/0/ 1 2
2 twtwtwrms
twtwrms
ttvtvtGttv
tA
Homogeneous space:
Layered space:
tvtGr
tA 1
Raw field records from land survey
Rapid decay in amplitudes at late times
Corrected field records from a land survey
Restored amplitudes at late times by correcting for geometrical spreading(unfortunately ambient noise also has been strengthened
Yilmaz, 1987
Normalization of amplitude for a certain time sample in a certain time window (not for the whole trace)
Advantage:
Amplification of Amplitudes for larger travel timesDisadvantage:
Shadow effectCan lead to amplification of noise
All traces are more equal which is needed for further processing (Stacking: summation of different traces)
No physical base for amplification
AGC - Automatic Gain Control
Different AGC functions
Numbers on top indicate gain window sizes in millisecondsSmall time gates: loss of signal character by boosting small amplitude zones
Compensation for losses and geometrical spreading:
Advantage:Partly based on physicsKnown function: original data can be recovered
Disadvantage:Results depend strongly on used gain function
Programmable Gain function
atnektAtA 0)(
Calculation of decay of amplitude and determine a Gain function
programmed gain curve
Yilmaz, 1987
Four different PGC functions
Scale factors are indicated by the circled numbers at the times of application
All traces are normalized using a certain amplitude:
RMS
Median value
Maximum Value
Advantage:All traces are more equal which is needed for further processing (Stacking: summation of different traces)
Disadvantage:No physical base for amplificationNo equalisation of losses with timeLarge value in a trace can dominate
Trace balancing
Common shot-gathers just after demultiplexingCorrected for wavefront divergence
After trace balancing
Yilmaz, 1987
Field record from marine survey
Raw field record +Geometric spreading correction
+Trace balancing
Yilmaz, 1987